6.2 HST Visits

6.2.1 Defining New Visits and Optimizing Scheduling Efficiency and Flexibility

Guidelines and Rules:

The following guidelines were put in place to ensure scheduling efficiency and flexibility, and to maximize the number of scheduling opportunities during the HST observing cycle. We recommend that:

• Visits should not exceed five (5) orbits.
• Exposures should be ordered and grouped such that instrument overheads occur between orbital viewing periods in multi-orbit visits.
• Changes in HST pointing should not exceed ~2 arc minutes. This can be due to an explicit change in target position (e.g. POS TARG, pattern, aperture change) or the use of a new target. Changes larger than 2 arc minutes introduce major slews which can be accommodated but, only if science goals dictate and conditions allow it.

A new visit is required if any of the following conditions occur:

1. a change in HST pointing of greater than ~1 degree.
2. the interval of time between repeated or periodic exposures creates an empty visibility period (an orbit with no exposures).
3. there is a required change in telescope roll orientation between observations.

A more complete explanation of the rationale behind these guidelines and rules can be found at the Visit Size Recommendations Web page.

The practical implementation of these guidelines is dictated by the details of the telescope and instrument operating characteristics. Proposers should use the Phase I documentation and proposal tools to gain insight into how well a proposed observing scenario satisfies each of the guidelines.

In general, the rule of thumb is that "smaller is better". Thus, smaller visit durations, target separations, and instrument configurations are better, where "better" refers to telescope scheduling efficiency and flexibility. STScI will work with observers (in Phase II) to find the best observing strategy that satisfies the science goals while following these guidelines as closely as possible.

6.2.2 Instrument Specific Limitations on Visits

For all SIs except WFPC2, there are instrument-specific restrictions on the definition of a visit.

ACS: Data Volume Constraints

If ACS data are taken at the highest possible rate (~ 5 WFC images per orbit) for several consecutive orbits, it is possible to accumulate data faster than it can be transmitted to the ground, even when using both HST data transmitters. High data volume proposals will be reviewed and on some occasions, users may be requested to divide the proposal into different visits or consider using subarrays. Users can achieve higher frame rates by using subarrays, at the expense of having a smaller field of view; see the ACS Instrument Handbook for details.

FGS: Astrometry

For astrometric observations using FGS1R, each individual set (consisting of target object and reference objects) may be contained in one visit if there is no telescope motion made during the sequence.

Coronagraphy

In two-gyro mode, all ACS or NICMOS coronagraphic observations will be single visits using the full orbit for science observations. Two images of the same target within the same orbit, with a roll of the telescope between observations, will not be possible.

As an extra insurance policy, coronagraphic observers may want to consider adding an extra orbit for each new pointing. Thermal changes in the telescope are likely to be significantly smaller in the second and subsequent orbits on a target than they are in the first orbit.

Coronagraphic observations requiring particular telescope orientations (e.g., positioning a companion or disk between diffraction spikes) are time-critical and must be described in the 'Special Requirements' section of a Phase I proposal (see Section 9.3 of the Call for Proposals).

STScI will provide standard calibration reference files, flat fields and darks, which will be available for calibration purposes. Contemporary reference files in support of coronagraphic observations are not solicited or normally approved for GO programs, but coronagraphic observers who can justify the need for contemporary calibration observations must include the additional orbit request in the Phase I proposal. Acquisition of bright targets for which an onboard ACQ with NICMOS will not be feasible requires the observer to obtain flat field observations to locate the coronagraphic hole. This implies adding one or more orbits to the total time requested. All calibration data regardless of the program are automatically made public.